1. Field of the Invention
This invention relates to a coating apparatus for coating paper or board or for applying ink in a printing machine or the like.
2. Information Disclosure Statement
In the prior art coaters, an applicator roll disposed within a coater pan has been proposed. The applicator roll counter-rotates relative to a backing roll such that coating material within the pan is carried by the applicator roll towards an application zone, or gap, disposed between the applicator roll and the backing roll. The prior art proposal includes a coating shoe which separates the coating material into two flow paths. The first flow path guides coating material between the shoe and the web extending around the backing roll. The second flow path follows the surface of the applicator roll. Excess coating material from the first flow path overflows through an overflow pipe and is recirculated.
In the prior art proposal, the gap between the applicator roll and the web extending around the backing roll ranges from 0.2 to 0.5 millimeters and the applicator roll usually rotates at a speed which is 10 to 20 percent less than the speed of the web.
As the coating material is lifted by rotation of the applicator roll towards the web, such rotation of the applicator roll generates a wedge pressure which builds up during the passage of the web through the gap between the backing roll and the applicator roll. The coating material is split into the aforementioned two flow paths, the first flow path running adjacent to the web whereas the second flow path follows the applicator roll. The point at which the aforementioned flow paths diverge constitutes a split point. Excess coating material is scraped off after the split point in order to maintain the correct amount of coating, or target coating, on the web. Such excess coating material is returned through the aforementioned overflow pipe to the coater pan.
In the prior art proposals, coated film thicknesses applied to the paper web have been in the order of 200 to 300 microns and the final film thickness after scraping the excess coating material from the web was about 20 microns. The outer surface of the coating material downstream relative to the gap and prior to removal of excess coating material by a blade constitutes an unstable, free boundary surface due to a negative pressure produced therein. Accordingly, pressure variations exist in a cross-machine direction along the width of the roll which result in the formation of coating patterns of the coating material. These coating patterns include longitudinal stripes which include a wavy, lateral cross-sectional configuration.
According to the present invention, the foregoing striped coating patterns generated by the prior art blade type coaters have reduced the quality of the resultant coated paper webs. Such coated webs not only include streaks but also split marks. The aforementioned free boundary surface is unavoidable in coaters employing two rolls. More specifically, the aforementioned striped coating pattern is caused by pressure variations in a cross-machine direction in the coating material at the aforementioned gap, or nip section. These pressure variations often result from air entrapped within the nip section.
Additionally, in the prior art two-roll coater, the pressure becomes high in the nip section and the coating material tends to penetrate the web resulting in staining of the backing roll. Therefore, it is a primary objective of the present invention to overcome the aforementioned inadequacies of the prior art proposals and to provide a coater which makes a considerable contribution to the coating art.
Another object of the present invention is the provision of a coating apparatus which includes a backing roll and an applicator roll and a nip roll movably cooperating with the backing roll for defining therebetween a nip section. The nip section is disposed downstream relative to the gap defined between the applicator and backing rolls such that the web extends progressively through the gap and through the nip section so that the coating material is applied to the surface of the web during passage of the web through the gap and nip section.
Another object of the present invention is the provision of a coating apparatus in which the gap between the backing roll and the applicator roll is variable.
Another object of the present invention is the provision of a coater apparatus in which a coater pan for the reception therein of coating material defines an opening such that the applicator roll is partially disposed within the pan so that when the applicator roll is rotated, the coating material within the pan is carried by the applicator roll from the pan towards the gap and nip section.
Another object of the present invention is the provision of a coating apparatus which includes means for moving the pan towards the backing roll.
Another object of the present invention is the provision of a coating apparatus in which an arm means rotatably supports a rotatable nip roll such that the nip roll rides on the applicator roll.
Another object of the present invention is the provision of a coating apparatus in which the nip roll is rotated by means of power means such that the nip roll rotates in the same direction of rotation as the applicator roll for applying a smooth layer of the coating material onto the web during passage of the web through the nip section, the nip roll being rotated at a speed of rotation such that accumulation of coating material thereon is inhibited.
Another object of the present invention is the provision of a coating apparatus in which the diameter of the nip roll is less than the diameter of the applicator roll and the backing roll such that during passage of the web through the nip section, the nip pressure applied between the nip roll and the backing roll is insufficient to cause penetration of the web and the resultant staining of the backing roll.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.